This application relates to telecommunication systems and, more specifically, to a method and apparatus for improving the functionality of a telephone hybrid circuit. The invention provides a means for sensing the communication signal on the bidirectional telephone line pair by incorporating a sense winding in a coupling transformer. Several benefits are provided by the apparatus using the sense winding including an improved signal-to-noise ratio at the receiver output port.
A variety of two wire to four wire conversion circuits are used extensively in telecommunication networks. The conversion circuits typically exemplified are hybrid circuits. Hybrid circuits as used herein may also be referred to as analog echo canceller circuits. FIG. 1 is provided to illustrate the functionality of a hybrid circuit.
A hybrid circuit typically has two half-duplex paths, a transmit pair 118 and a receive pair 116, and a full-duplex bidirectional pair 112 of wires. The bidirectional pair may be, for example, a pair of telephone wires coupled from a customer location to a telephone central office (CO) or other facility. The bidirectional pair serves as a transmission channel for a signal from the customer location to a CO and for a signal from the CO to the customer location. Hence at the CO, where a hybrid circuit is used, the bidirectional pair has a receive signal from the customer location and a transmit signal from the CO. The hybrid circuit provides a means for separating the transmit signal and the receive signal at the CO. Persons working in the telecommunication field would appreciate the hybrid circuit can also be used in data communication equipment at a customer location or elsewhere within telecommunication networks.
Referring again to FIG. 1 there is shown a hybrid circuit. The hybrid circuit, as illustrated, is a four port device having a bidirectional port, a receive port, a transmit port, and a balancing impedance port. The bidirectional port is coupled to a bidirectional channel for bidirectional signal flow, i.e., transmit and receive signals flow on the bidirectional channel. The transmit port is the input for a transmit signal which is coupled by a transmit pair of wires. A portion of the transmit signal is coupled to the bidirectional channel for transmission to a far end location. The receive port is coupled to the bidirectional channel and receives a far end signal which is transmitted from the far end location. The receive port therefore contains a receive signal, where the receive signal is typically an attenuated version of the signal from the far end location. The balancing impedance port is coupled to an impedance approximately equal to the impedance of the bidirectional channel.
An ideal hybrid circuit has no energy transferred from the transmit port to the receive port while maximizing energy from the transmitter to the bidirectional port and from the far end through the bidirectional port to the receiver. A figure of merit called the transhybrid loss is used as a measure of the amount of transmit signal contained in the receive signal. It is also important to consider the efficiency with which the transmit signal is transferred to the bidirectional port, and the receive signal is received from the bidirectional port when evaluating the overall performance of a hybrid circuit.
An example of a conventional hybrid circuit is a passive circuit using specially wound transformers, such as described in Transmission Systems for Communications by Members of the Technical Staff at Bell Telephone Laboratories, 1981. Conventional hybrid circuit designs include circuits with and without transformers and may use summing amplifiers for signal canceling, as opposed to the canceling magnetic flux arrangements of the hybrid cited in the above reference. Still other conventional circuits, such as the one disclosed by Hirohisa in Japanese Patent Publication 06068346, recognize the need to canceling out the effects of internal resistance variations due to temperature variations of the transformer windings.
One objective of the present invention is to increase the signal-to-noise ratio (SNR) at the output of a receiver. The receive SNR depends upon the amount of far end (desirable) signal appearing at the receiver, as well as the amount of near end (undesirable) signal appearing at the receiver. An increase in SNR will provide a better bit error ratio and can also allow for an increase in transmission distance. In some data communication systems around a one dB increase in SNR will allow for an additional 500 feet of cable between transceivers, i.e., between the near end and far end locations.
Another objective of the present invention is to remove the DC response ambiguity caused by variations of winding resistance in hybrid circuits using transformers. The variations of winding resistance between various transformers of a given kind and with temperature typically causes the transfer function of the hybrid circuit using a transformer to change at low frequencies. Hence there is a need to avoid the DC response ambiguity caused by temperature and component variations.
In some hybrid circuits having transformers it is desirable that the pick-off signal be at a voltage level independent of either the far end voltage or the near end voltage. The circuit of the present invention, having a separate sense winding on the transformer, provides a means for independently adjusting the level of the pick-off signal by adjusting the number of turns in the sense winding.
Another objective of the present invention is to reduce the effects transformer leakage inductance has on the replica transfer function. A reduction in these effects allows a hybrid circuit having a transformer to operate over a wider range of frequencies, thereby providing better hybrid performance. Further, it renders the hybrid circuit relatively insensitive to changes in leakage inductance with different transformers.
Because analog systems in telecommunications systems may operate with a variety of common-mode voltages and power supply voltages, it is sometimes useful to have DC isolation between circuits. The present invention provides a means for providing DC isolation.
The above objectives indicate there is a need for an improved method and apparatus for providing hybrid coupling. Further the apparatus and method should be cost effective and have parameters that may be changed to meet the needs of individual users. A sense winding on a transformer arranged as a coupling element serves to meet the above objectives.
Thus, in accordance with a preferred embodiment of the present invention, an apparatus is provided for canceling a near end signal from a far end signal in a communication system having a bidirectional transmission medium. The apparatus includes: a transformer first winding for conveying the near end signal to the bidirectional transmission medium; a transformer second winding coupled to the first winding and the transmission medium for outputting the near end signal onto the bidirectional transmission medium; and a transformer sense winding galvanically isolated from both the first and second windings but coupled to the bidirectional transmission medium for receiving the far end signal from the bidirectional transmission medium and generating a sense winding output containing energy corresponding to both the near end signal and the far end signal. The apparatus further includes a replica network for generating a replica of the near end signal, and a receiver for combining the sense winding output and the replica to provide a receiver output signal that has a substantially reduced amount of near end energy.
In another aspect of the present invention, a method of providing hybrid functionality is disclosed for a system in which near end and far end signals are transmitted and received on a bidirectional communications medium. The method includes the steps of: transmitting a near end signal on the bidirectional communications medium; receiving, at a transformer sense winding having an independently selectable number of turns and an independently selectable DC bias voltage, the far end signal from the bidirectional communications medium; providing, from the sense winding to a receiver, a sense winding output signal having energy corresponding to both the near end and far end signals; generating a replica of the near end signal; providing the replica to the receiver; and combining, at the receiver, the sense winding output signal and the replica to generate a receiver output signal that has a substantially reduced amount of transmit energy.